The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun. Of the many objects that orbit the Sun, most of the mass is contained within eight relatively solitary planets whose orbits are almost circular and lie within a nearly flat disc called the ecliptic plane. The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, the gas giants, are substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are composed largely of ices, such as water, ammonia and methane, and are often referred to separately as "ice giants".
The Solar System is also home to a number of regions populated by smaller objects. The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc; linked populations of trans-Neptunian objects composed mostly of ices such as water, ammonia and methane. Within these populations, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are recognized to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets. In addition to thousands of small bodies in those two regions, several dozen of which are considered dwarf-planet candidates, various other small body populations including comets, centaurs and interplanetary dust freely travel between regions. Six of the planets and three of the dwarf planets are orbited by natural satellites, usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other particles.
The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc. The Oort cloud, which is believed to be the source for long-period comets, may also exist at a distance roughly a thousand times further than the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of interstellar wind. The Solar System is located within one of the outer arms of Milky Way galaxy, which contains about 200 billion stars. In astronomy, the main source of information about celestial bodies and other objects is visible light or more generally electromagnetic radiation. Observational astronomy may be divided according to the observed region of the electromagnetic spectrum. Some parts of the spectrum can be observed from the Earth's surface, while other parts are only observable from either high altitudes or outside the Earth's atmosphere.
Hubble's high resolution images of the planets and moons in our Solar System can only be surpassed by pictures taken from spacecraft that actually visit them. Hubble even has one advantage over these probes: it can look at these objects periodically and so observe them over much longer periods than any passing probe could. Hubble has observed all the planets in our Solar System, apart from Earth and Mercury. Earth is far better studied by geologists on the ground and specialised probes in orbit. Hubble can’t observe Mercury as it is too close to the Sun, whose brightness would damage the telescope’s sensitive instruments. This is crucial for the study of planetary atmospheres and geology.
Our Sun is not unique in the universe. It is a common middle-sized yellow star which scientists have named Sol. This is why our system of planets is called the Solar System. There are trillions of other stars in the Universe just like it. Many of these stars have their own systems of planets, moons, asteroids, and comets. The Sun is the Solar System's star, and by far its chief component. Its large mass produces temperatures and densities in its core great enough to sustain nuclear fusion, which releases enormous amounts of energy, mostly radiated into space as electromagnetic radiation, peaking in the 400–700 nm band of visible light. The Sun is classified as a type G2 yellow dwarf, but this name is misleading as, compared to the majority of stars in our galaxy, the Sun is rather large and bright. Stars are classified by the Hertzsprung–Russell diagram, a graph that plots the brightness of stars with their surface temperatures. Generally, hotter stars are brighter. Stars following this pattern are said to be on the main sequence, and the Sun lies right in the middle of it. However, stars brighter and hotter than the Sun are rare, while substantially dimmer and cooler stars, known as red dwarfs, are common, making up 85 percent of the stars in the galaxy. Evidence suggests that the Sun's position on the main sequence puts it in the "prime of life" for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is growing brighter; early in its history it was 70 percent as bright as it is today.
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The planet Mercury is the closest of the planets to the Sun. Because this planet lies so close to the Sun, and as a result somewhat near to the Earth, it is visible to observers on Earth in the night time sky. Because of this, Mercury has become apart of the mythology and legend of almost every culture throughout the history of the Earth. This planet is often called a morning star. This is because Mercury shines brightly in the early morning, just before the sun rises. It has also been called an evening star for the same reason. Mercury often is visible for a brief period of time just after the Sun sets. Mercury has no moons. Mercury, the innermost planet of the solar system, is a little bigger than the Earth's Moon. The surface of the planet is covered with craters, like the Moon, but temperatures there can reach over 800ºF because Mercury is so close to the Sun and rotates so slowly. Scientists believe that the interior structure of Mercury includes a metallic core, an intermediate rocky layer, and a thin brittle crust. The composition of Mercury is probably high in iron, although surface features indicate that volcanic activity once existed at the surface. There is little evidence of motions near the surface of the planet now, although at earlier times during Mercury's evolution the surface was much more active. We know relatively little about Mercury, compared to most of the other planets, because it is relatively difficult to see and only one spacecraft has studied the planet. Observations of Mercury from ground-based observers and the Mariner 10 spacecraft have not shown evidence of a significant atmosphere compared to other planets. Mariner 10 did observe a tiny amount of helium 1000 km above the surface, but this is probably produced by capturing particles from the solar wind and radioactive decay of Mercury's crust.
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The planet Venus has long been one of the most misunderstood of all the inner planets. Like the Earth, Venus has an atmosphere. However, this atmosphere is far thicker than that of the Earth, making it difficult for modern science to penetrate. Because we cannot see the surface of the planet Venus visually, there is much we still do not know about how this planet looks, and what it is like. However, using special instruments and probes scientists have in recent years unlocked many of the secrets long hidden by this mysterious world. Venus has no moons. Venus is the second planet from the Sun, and is Earth's neighbor in the solar system. Venus is the brightest object in the sky after the Sun and the Moon, and sometimes looks like a bright star in the morning or evening sky. We can't see the surface of the planet because it has a very thick atmosphere filled with clouds that strongly reflect sunlight. Observations of Venus in the ultraviolet show cloud features that relate to characteristics of the planet's atmosphere. Venus is slightly smaller than Earth, and considering its neighboring position in the solar system might be expected to be similar to Earth. We think that the internal structure of Venus is similar to Earth, with a metallic core, rocky mantle, and crust. The atmosphere of Venus produces hostile conditions at the planet's surface, where temperatures can reach more than 900ºF, atmospheric pressure is 90 times that at the Earth's surface, and clouds filled with sulfuric acid surround the planet. Nonetheless, space missions sent to Venus have managed to land on and photograph the surface while the Magellan spacecraft mapped the surface of the planet from above. These maps reveal a surface covered with craters, over 1600 major volcanoes, mountains, large highland terrains, and vast lava plains. The atmosphere is composed mainly of 96% carbon dioxide, 3.5% nitrogen, and less than 1% is made up of carbon monoxide, argon, sulfur dioxide, and water vapor.
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Our planet is an oasis of life in an otherwise desolate universe. The Earth's temperature, weather, atmosphere and many other factors are just right to keep us alive. The Earth has one moon. Its name is Luna. Earth, the largest and densest rocky planet, was formed about 4.6 billion years ago. The Earth's interior is divided into four layers which is typical of rocky planets. Each layer has different characteristics and is made of different elements and minerals. There are many different types of features on Earth’s surface due to the complexity of our planet. The surface is unique from the other planets because it is the only one which has liquid water in such large quantities. Water forms some features of Earth's surface such as rivers, oceans, beaches and lakes. Other surface features, such as mountains, earthquakes and volcanoes, are formed when large pieces of the Earth’s outer layer move slowly by plate tectonics.
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Mars excites scientists because its mild temperament is more like the Earth's than any of the other planets. Evidence suggests that Mars once had rivers, streams, lakes, and even an ocean. As Mars' atmosphere slowly depleted into outer space the surface water began to permanently evaporate. Today the only water on Mars is either frozen in the polar caps, or underground. Mars has two moons, their names are Deimos and Phobos. The uniquely red global surface of Mars is marked by many interesting features - some like those on the Earth and others strangely different. The reddish color is caused by rust (iron oxide) in the soil. Some of these features are; volcanoes, canyon systems, river beds, cratered terrain, and dune fields. Of these features, the most interesting includes the apparently dead volcano Olympus Mons, which rises 75,000 ft above the surrounding plains and is the highest known peak in the Solar System. Valles Marineris is a giant canyon system that runs about 2,500 miles across the surface of the planet and reaches depths 4 miles.
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Jupiter's great red spot is where a giant storm has been raging for at least 300 years. This storm's super hurricane winds blow across an area larger than the Earth. This giant planet has no solid surface. Under its atmosphere is a large liquid ocean of hydrogen and water. What lies in between that ocean and the atmosphere? Actually there is no in between. The atmosphere slowly gets thicker and thicker until it becomes part of the ocean. In other words Jupiter's ocean has no surface. The sky slowly becomes the ocean. Jupiter has at least 63 moons. Some of heir names are Metis, Adrastea, Amalthea, Thebe, Io, Europa, Ganymede, Callisto, Sinope, Leda, Himalia, Lysithea, Elara, Ananke, Carme, and Pasiphae. The Giant planets do not have the same layered structure that the terrestrial planets do. Their evolution was quite different than that of the terrestrial planets, and they have less solid material inside. Jupiter's interior composition is primarily that of simple molecules such as hydrogen and helium, which are liquids under the high pressure environments found in the interiors of the outer planets, and not solids. Motions in the interior of Jupiter contribute in a very special way to the development of the powerful and extensive magnetosphere of Jupiter. Heat generated within Jupiter contributes to the unusual motions of the atmosphere.
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Saturn is a favorite object for many observers. Its beautiful rings are 169,800 miles wide. In many ways Saturn is similar to Jupiter, but it is much smaller. Under the clouds of methane and helium the sky gradually turns into liquid until it becomes a giant ocean of liquid chemicals. Saturn has 34 moons. Some names of Saturn's moons are Pan, Atlas, Prometheus, Pandora, Epimetheus, Janus, Mimas, Enceladus, Tethys, Telesto, Calypso, Dione, Helene, Rhea, Titan, Hyperion, Iapetus, and Phoebe. The Giant planets do not have the same layered structure that the terrestrial planets do. Their evolution was quite different than that of the terrestrial planets, and they have less solid material inside. Saturn's interior composition is primarily that of simple molecules such as hydrogen and helium, which are liquids under the high pressure environments found in the interiors of the outer planets, and not solids. Motions in the interior of Saturn contribute in a very special way to the development of the powerful and extensive magnetosphere of Saturn. Heat generated within Saturn contributes to the unusual motions of the atmosphere. The dramatic appearance of Saturn stems mainly from the spectacular rings. What is visible of the atmosphere is much less dramatic. The clouds of Saturn are much less colorful than those of Jupiter. This is because the composition of Saturn's atmosphere includes more sulfur. This adds to Saturn's overall yellow appearance. When you look closely, however, Saturn's atmosphere is just as fierce as that of Jupiter.
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Uranus is very odd. Unlike all the other planets and most of the moons in our Solar System Uranus spins on its side. It is believed that long ago a very large object smashed into this planet. The crash was so powerful that it completely changed the direction of Uranus' spin. Uranus may have an ocean of water beneath its clouds. It has a large rocky core, and because of the tremendous pressure could possibly contain trillions of large diamonds. Uranus is almost identical to the planet Neptune. Uranus has 27 moons. Five of these moons are large, and the rest are smaller. Some of their names are Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, Puck, Miranda, Ariel, Umbriel, Titania, and Oberon. Uranus's interior composition is primarily that of methane ice. Motions in the interior of Uranus contribute to the formation of the magnetosphere of Uranus. Heat generated within Uranus contributes to the unusual motions of the atmosphere. The planet appears to be blue-green because the atmosphere absorbs the red wavelengths of the visible spectrum. The uniformity of the planet's appearance confirms that the planet's atmosphere is composed almost solely of one element, methane gas.
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Neptune has a giant storm much like the storm on Jupiter. This storm is often called The Great Dark Spot. We do not know how long this storm has been active, because it is so far away that we could not get a good view of Neptune until modern times when we began to send robot explorers. For many centuries people did not know that this planet even existed. It was discovered by Johann Galle and Heinrich D'Arrest in 1846. Neptune has six rings which circle the planet. Neptune has 13 moons that we know of. Because Neptune is so far away, it is difficult to see any of these worlds. There are probably many more moons orbiting this blue planet which we have not yet discovered. The names of the known moons are: Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, Triton, and Nereid. Neptune's interior composition is primarily that of methane ice. Neptune's atmosphere shows a striped pattern of clouds. This cloud pattern is very similar to that of Jupiter and Saturn. Neptune even has a Great Dark Spot similar to Jupiter's Great Red Spot.
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Pluto is smaller than 7 of the moons in the Solar System. Because it is so small many scientists don't consider it a planet at all. In 1999 a group of scientists attempted to redesignate Pluto as a comet. In August 2006 Pluto's status was officially changed from planet to dwarf planet. Pluto has three moons. The largest is named Charon. Charon is only slightly smaller than Pluto.
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A comet is an icy, small Solar System body that warms and begins to release gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, and sometimes a tail of gas and dust gas blown out from the comet.
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Until recently, impacts by extraterrestrial bodies were regarded as an interesting but, perhaps, not an important phenomenon in the spectrum of geological process affecting the Earth. Our concept of the importance of impact processes, however, has been changed radically through planetary exploration, which has shown that virtually all planetary surfaces are cratered from the impact of interplanetary bodies. Although the number of known impact craters on Earth is relatively small, the preserved sample is an extremely important resource for understanding impact phenomena. They provide the only ground-truth data currently available and are amenable to extensive geological, geophysical and geochemical study. Our home is in a shooting gallery, and we do get hit daily with something. Some are easy to see from space, and some are not so easy to see because nature reclaims the land quickly.
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